--- a/build/win32/mozconfig.vs2015-win64
+++ b/build/win32/mozconfig.vs2015-win64
@@ -1,10 +1,11 @@
 if [ -z "${VSPATH}" ]; then
-    VSPATH="$(cd ${topsrcdir} && pwd)/vs2015u1"
+    TOOLTOOL_DIR=${TOOLTOOL_DIR:-$topsrcdir}
+    VSPATH="$(cd ${TOOLTOOL_DIR} && pwd)/vs2015u1"
 fi
 
 export WINDOWSSDKDIR="${VSPATH}/SDK"
 export WIN32_REDIST_DIR="${VSPATH}/VC/redist/x86/Microsoft.VC140.CRT"
 export WIN_UCRT_REDIST_DIR="${VSPATH}/SDK/Redist/ucrt/DLLs/x86"
 
 export PATH="${VSPATH}/VC/bin/amd64_x86:${VSPATH}/VC/bin/amd64:${VSPATH}/VC/bin:${VSPATH}/SDK/bin/x86:${VSPATH}/SDK/bin/x64:${VSPATH}/DIASDK/bin:${PATH}"
 export PATH="${VSPATH}/VC/redist/x86/Microsoft.VC140.CRT:${VSPATH}/VC/redist/x64/Microsoft.VC140.CRT:${VSPATH}/SDK/Redist/ucrt/DLLs/x86:${VSPATH}/SDK/Redist/ucrt/DLLs/x64:${PATH}"

--- a/build/win64/mozconfig.vs2015
+++ b/build/win64/mozconfig.vs2015
@@ -1,10 +1,11 @@
 if [ -z "${VSPATH}" ]; then
-    VSPATH="$(cd ${topsrcdir} && pwd)/vs2015u1"
+    TOOLTOOL_DIR=${TOOLTOOL_DIR:-$topsrcdir}
+    VSPATH="$(cd ${TOOLTOOL_DIR} && pwd)/vs2015u1"
 fi
 
 export WINDOWSSDKDIR="${VSPATH}/SDK"
 export WIN32_REDIST_DIR=${VSPATH}/VC/redist/x64/Microsoft.VC140.CRT
 export WIN_UCRT_REDIST_DIR="${VSPATH}/SDK/Redist/ucrt/DLLs/x64"
 
 export PATH="${VSPATH}/VC/bin/amd64:${VSPATH}/VC/bin:${VSPATH}/SDK/bin/x64:${VSPATH}/VC/redist/x64/Microsoft.VC140.CRT:${VSPATH}/SDK/Redist/ucrt/DLLs/x64:${VSPATH}/DIASDK/bin/amd64:${PATH}"